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D-Glucose transport in piglet jejunal brush-border membranes: insights from a disease model
Abstract:
We measured glucose transport in jejunal brush-border membrane vesicles isolated from piglets with acute viral diarrhea, comparing our results with those from control animals. Characterization of membranes from both study groups demonstrated comparable purity and integrity. In the presence of an inwardly directed Na SCN gradient, D-glucose accumulated in control vesicles to a concentration several times the 60-min equilibrium level. "Overshooting" uptake was much lower and more gradual in vesicles from 40-h transmissible gastroenteritis (TGE)-infected pigs compared with control pigs. Equilibrium kinetic studies, in which gramicidin was used to clamp membrane potential at zero, demonstrated a pattern of Na-dependent D-glucose transport in 40-h TGE-infected membranes that differed greatly from the control pattern. From an Eadie-Hofstee plot of stereospecific Na-dependent D-glucose uptake into control vesicles, a pattern suggesting two carrier populations emerged: one with a low-affinity, apparent Km equaling 52.63 +/- 13.81 mM and the other a high-affinity apparent Km equaling 3.92 +/- 0.24 mM for D-glucose. In 40-h TGE-infected membranes, the pattern conformed to a single line, suggesting a homogeneous population of low-affinity carriers, (Km = 37.03 +/- 1.92 mM), which did not differ from the low-affinity carriers seen in control animals. We conclude that the absence of the high-affinity D-glucose carriers in jejunal brush-border membrane is an important determinant of the defective glucose transport that characterizes viral diarrhea. Because previous studies have strongly suggested that in acute TGE diarrhea the epithelium is composed of relatively undifferentiated crypt-type cells, we speculate that high-affinity D-glucose carriers are lacking in normal crypt epithelial cells and that they are incorporated into brush-border membranes of jejunal enterocytes as the cells differentiate in the course of their migration from crypt to villus.